Tooth mineral is composed predominantly of calcium hydroxyapatite, Ca10(PO4)6(OH)2, which may be partially substituted with anions such as carbonate or fluoride, and cations such as zinc or magnesium. Tooth mineral may also contain non-apatitic mineral phases such as octacalcium phosphate and calcium carbonate.
Tooth loss may occur as a result of dental caries, which is a multifactorial disease where bacterial acids such as lactic acid produce sub-surface demineralisation that does not fully remineralise, resulting in progressive tissue loss and eventually cavity formation. The presence of a plaque biofilm is a prerequisite for dental caries, and acidogenic bacteria such as Streptococcus mutans may become pathogenic when levels of easily fermentable carbohydrate, such as sucrose, are elevated for extended periods of time.
Even in the absence of disease, loss of dental hard tissues can occur as a result of acid erosion and/or physical tooth wear; these processes are believed to act synergistically. Exposure of the dental hard tissues to acid causes demineralisation, resulting in surface softening and a decrease in mineral density. Under normal physiological conditions, demineralised tissues self-repair through the remineralising effects of saliva. Saliva is supersaturated with respect to calcium and phosphate, and in healthy individuals saliva secretion serves to wash out the acid challenge, and raises the pH so as to alter the equilibrium in favour of mineral deposition.
Dental erosion (i.e. acid erosion or acid wear) is a surface phenomenon that involves demineralization, and ultimately complete dissolution of the tooth surface by acids that are not of bacterial origin. Most commonly the acid will be of dietary origin, such as citric acid from fruit or carbonated drinks, phosphoric acid from cola drinks and acetic acid such as from vinaigrette. Dental erosion may also be caused by repeated contact with hydrochloric acid (HCl) produced in the stomach, which may enter the oral cavity through an involuntary response such as gastroesophageal reflux, or through an induced response as may be encountered in sufferers of bulimia.
Tooth wear (i.e. physical tooth wear) is caused by attrition and/or abrasion. Attrition occurs when tooth surfaces rub against each other, a form of two-body wear. An often dramatic example is that observed in subjects with bruxism, a grinding habit where the applied forces are high, and is characterised by accelerated wear, particularly on the occlusal surfaces. Abrasion typically occurs as a result of three-body wear and the most common example is that associated with brushing with a toothpaste. In the case of fully mineralised enamel, levels of wear caused by commercially available toothpastes are minimal and of little or no clinical consequence. However, if enamel has been demineralised and softened by exposure to an erosive challenge, the enamel becomes more susceptible to tooth wear. Dentin is much softer than enamel and consequently is more susceptible to wear. Subjects with exposed dentin should avoid the use of highly abrasive toothpastes, such as those based on alumina. Again, softening of dentin by an erosive challenge will increase susceptibility of the tissue to wear.
Dentin is a vital tissue that in vivo is normally covered by enamel or cementum depending on the location i.e. crown versus root respectively. Dentin has a much higher organic content than enamel and its structure is characterised by the presence of fluid-filled tubules that run from the surface of the dentin-enamel or dentin-cementum junction to the odontoblast/pulp interface. It is widely accepted that the origins of dentin hypersensitivity relate to changes in fluid flow in exposed tubules, (the hydrodynamic theory), that result in stimulation of mechanoreceptors thought to be located close to the odontoblast/pulp interface. Not all exposed dentin is sensitive since it is generally covered with a smear layer; an occlusive mixture comprised predominantly of mineral and proteins derived from dentin itself, but also containing organic components from saliva. Over time, the lumen of the tubule may become progressively occluded with mineralised tissue. The formation of reparative dentin in response to trauma or chemical irritation of the pulp is also well documented. Nonetheless, an erosive challenge can remove the smear layer and tubule “plugs” causing outward dentinal fluid flow, making the dentin much more susceptible to external stimuli such as hot, cold and pressure. As previously indicated, an erosive challenge can also make the dentin surface much more susceptible to wear. In addition, dentin hypersensitivity worsens as the diameter of the exposed tubules increases, and since the tubule diameter increases as one proceeds in the direction of the odontoblast/pulp interface, progressive dentin wear can result in an increase in hypersensitivity, especially in cases where dentin wear is rapid.
Loss of the protective enamel layer through erosion and/or acid-mediated wear will expose the underlying dentin, and are therefore primary aetiological factors in the development of dentin hypersensitivity.
It has been claimed that an increased intake of dietary acids, and a move away from formalised meal times, has been accompanied by a rise in the incidence of dental erosion and tooth wear.
In view of this, oral care compositions which can help prevent dental erosion and tooth wear, in addition to dental caries, would be advantageous.
Oral care compositions comprising a source of fluoride ions have been known for many years for combating dental caries. Fluoride ions are known to inhibit plaque bacteria that can cause plaque acid. Fluoride ions are also known to enhance remineralisation and to decrease demineralisation of dental enamel, thereby strengthening dental enamel from acidic challenges.
In more recent years, oral care compositions comprising a source of fluoride ions have also been marketed for combating dental erosion. Some dentifrice compositions have been especially formulated to maximise both the availability of fluoride ions in the composition and their uptake by dental enamel, so to strengthen teeth from both dietary and plaque acidic challenges. It is suggested that such dentifrices suitably do not contain calcium salts.
Attempts have been made over many years to maximise the efficacy of fluoride ions, in strengthening dental enamel, by including a source of calcium and phosphate ions to supplement the natural remineralisation provided by such ions already present in saliva.
However, formulating a source of fluoride ions together with a calcium (phosphate) compound is technically challenging given that the presence of calcium ions together with a source of fluoride ions, can cause the precipitation of insoluble calcium fluoride, thereby significantly reducing the availability of fluoride in an oral care composition. Various solutions to this problem have been suggested including the incorporation of an antinucleating agent or an alkali metal phytate together with the calcium and fluoride ions or the separation of a calcium compound from a source of fluoride ions either by means of a physical barrier in a two phase aqueous composition or by formulating the ingredients in a single phase anhydrous system.
Known anti-carious remineralizing gels, toothpastes and dentifrices comprising from about 0.5 to 10% by weight of α-tricalcium phosphate, tetracalcium phosphate or monocalcium phosphate monohydrate, suitably present in a dry mixture to be reconstituted into a gel by the addition of water which may contain other components including a source of fluoride ions, so to prevent premature reaction of the calcium and fluoride ions.
Some oral care compositions comprise various partially water soluble calcium salts, such as calcium sulphate, together with a source of phosphate and fluoride ions. Such calcium salts are separated until use from the source of phosphate and fluoride ions either by being formulated in a single phase anhydrous system or by means of a physical barrier in a two phase aqueous composition. It is suggested that the oral care compositions preferably contain from about 0.05% to about 15.0% by weight, more preferably from about 0.10% to about 10.0% by weight of the calcium salt(s), from about 0.05% to about 15.0% by weight, more preferably from about 0.10 to about 10.0% by weight of the phosphate salt(s) and from about 0.01% to about 5.0%, more preferably from about 0.02% to about 2.0% by weight of the fluoride salt(s).
Other oral care compositions comprise low quantities (i.e. from 0.01 to 0.09% by weight of the composition) of various sparingly soluble rod-shaped nanoparticulate calcium compounds such as hydroxyapatite, fluorapatite or calcium fluoride. It is suggested that such compositions can also contain other calcium compounds, which need not be nanoparticulate, such as calcium glycerophosphate, or calcium containing abrasives such as chalk, calcium pyrophosphate or dicalcium phosphate dihydrate. Such compositions can also comprise a source of fluoride ions, preferably in an amount of 0.01 to 0.2% by weight.
There are oral care compositions that comprise nanoparticulate calcium fluoride for combating dental erosion and/or tooth wear. The nanoparticulate calcium fluoride may be present in an amount of 0.001 to 20.0% by weight of the total composition, suitably from 0.01 to 10%, for example from 0.1 to 5.0% by weight of the total composition. It is stated that the oral care composition may further comprise a source of soluble fluoride ions, which can be present in an amount to provide from 25 to 3500 ppm, preferably from 100 to 1500 ppm of fluoride ions.
There are known in the art anti-cariogenic oral hygiene compositions that comprise calcium glycerophosphate and from 0.08 to 7.6% by weight of sodium monofluorophosphate, the sodium monofluorophosphate and the calcium glycerophosphate being present in the composition in a weight ratio of 10:1 to 3:1.
Accordingly, there remains an electrochemically driven problem with fluoride easily bonding with calcium to form calcium fluoride, effectively removing both calcium and fluoride from bioavailability. The present novel technology addresses this need.